Two-way radios, which may be incorporated in wireless communication devices, are known to include an antenna, a transformer, an antenna switch, a receiver section, and a transmitter section. The antenna switch couples either the receiver section or the transmitter section to the antenna via the transformer. The transformer may be a transformer balun (balanced/unbalanced) and is generally used to convert single ended signals into differential signals and conversely to convert differential signals into single ended signals. For example, received RF signals via the antenna are converted into differential signals, which are provided to a low noise amplifier of the receiver section. Conversely, differential signals from the transmitter section are converted into single ended signals that are provided to the antenna.
As the demand for integrated radios increases, many attempts have been made to integrate transformers and/or transformer baluns onto radio frequency integrated circuits. However, such integration has been limited due to flux leakage, capacitive coupling limits, and significant series resistance. To reduce these limitations, advances have been made in transformer IC design including coplanar interleaved transformers, toroidal and concentric transformers, overlay transformers and symmetric coplanar transformers. Coplanar interleaved transformers have the primary and secondary windings interleaved on the same integrated circuit layer, where the primary and secondary windings are constructed of planer metal traces. While coplanar interleaved transformers reduces size and resistance and are widely used, they suffer from low quality (Q) factor, small coupling coefficients, and, if used as a balun, the center tap is often at an undesirable location, resulting in an asymmetric geometry. As is known, asymmetry of a transformer winding causes an imbalance in the resulting differential signal and/or an imbalance in the resulting single ended signal from a differential signal.
Toroidal and concentric transformers have the primary and secondary windings on several dielectric layers of an integrated circuit. Each layer includes one or more primary and secondary turns, where turns on different layers are coupled in series using vias. Each of the primary turns, on each layer, is constructed around the secondary turns on the same layer. While such toroidal and concentric transformers are well suited for multi-layer structures, they suffer from weak coupling, inconvenient center tap locations, and are asymmetrical.
Overlay transformers include a primary spiral inductor on a top layer and a secondary spiral inductor on a lower layer. Such transformers have high coupling coefficients and relatively small area; however, the secondary is asymmetrical creating a loading asymmetry.
Symmetric coplanar transformers include the primary and secondary windings on the same layer with interconnecting bridges on lower layers. While such transformers have high symmetry, they have weak magnetic coupling and are usually large for desirable inductor values.
Therefore, a need exists for an on-chip transformer balun that is small, provides reasonable inductance values, has a high quality factor, reduced resistance, and high coupling coefficient.